Monday, July 20, 2009

Keys to Lowering Reactor Costs: Advanced Materials

U have not thought about silicon carbide and carbon-carbon composites in some time, but i continue to be attracted by Dr. David LeBlanc's suggestion that lower cost materials like stainless steel be substituted for more exotic materials, with some losss of performance but lower costs. I suggested that by lowering LFTR costs, the low cost LFTR's, which I called " the Big Lots Reactor" could be used to produce peak load energy. I have even more recently offered the proposal that LFTR heated molten salt be stored and stored heated salt by used to provide heat for closed cycle gas turbines, during periods of peak demand. By using stored salts, small, part time LFTRs could be used to provide several times their rated output, capacity, during periods of peak demand. I am not sure who first made this ingenious proposal.

The cost inflation that is effecting the price of all new electrical generating facilities all over the world is a matter of serious concern. If we assume that anthropogenic global warming is going to require the replacement of virtually every carbon based energy source in the world with a carbon-free energy sources that means that most of the world's generating capacity must be replaced during the next 40 years. The last thing that we need right now is run away inflation of new generator costs. Advocates of renewable energy must face the same inflationary prospects as advocates of nuclear power. The inflation problems of renewable generating system are in some respects worse than the problems of nuclear systems, because renewables use more of the materials that are rapidly increasing in price.

One of the keys to cost control is the substitution of low cost materials for higher costs materials. Lower cost does not mean lower quality, and the use of potential substitutes could in fact lead to improved reactor efficiency. Both silicon carbide (SiC/SiC) composites and Carbon-Carbon composites have potential for reactor use. Carbon-carbon composites are particularly interesting because there is an important history of their use in the aircraft industry. Carbon-carbons are known to be light, strong and fatigue resistant. Manufacturers like Boeing have a good understanding of manufacturing techniques for carbon-carbons parts. Materials and parts manufacture costs with carbon-carbons would be lower and less subject to inflation than with metals. Other advantages of carbon carbons include excellent heat transport properties, making them potential materials for heat exchange, and tolerance for both fluoride salts and helium. Carbon-carbon composites can also withstand any temperature likely to be encountered inside a reactor. Thus the use of carbon-carbon composites is consistent with greater thermal efficiencies than are possible with the use of metal in reactor construction.

Some forms of carbon-carbon composites do not tolerate neutron radiation well. Thus is by no means certain that carbon-carbons can be used for reactor core structures. Material scientists have by no means given up on carbon-carbons in high neutron environments, and research continues. At the very least carbon-carbons can be used for reactor coolant piping, pumps and heat exchanges in LFTRs. If material scientists can solve the radiation tolerance problem carbon-carbons could be used in reactor cores structure as well.

It should be noted that carbon-carbons are potentially ideally suited for LFTR's, but if their neutron radiation problem can be solved, they could also be used to build reactor core structures for PBRs as well. The neutron resistant qualities of SiC/SiC composites have been studied and it would appear that some forms of SiC/SiC are fairly neutron resistant. Exactly how resistant remains to be seen. I have yet to see a definitive statement on the compatibility of SiC/SiCs with fluoride salts, yes, but in discussions of their use with fluoride salts, that is usually assumed. I have seen a statement made some 10 years ago about manufacturing difficulties with SiC/SiCs, but I am not aware if the problem was due to a lack of experience, or if it reflected an underlying problem with SiC/SiCs. It is not clear to what extent use of SiC/SiCs would lower reactor-manufacturing costs. However, its use might still be justified even if it proved more expensive, because it could enable reactors to operate at higher temperatures than would be possible with metal components. Thus if SiC/SiCs proved to be relatively expensive to manufacture, their cost might be repaid many times over by the added electrical output produced by greater thermal efficiencies.

Thus carbon-carbon composites are very promising materials for reactor structures involving movement of liquid salts outside reactors, and for heat exchanges. There use in reactor core structures cannot be discounted, but more research and development are required. SiC/SiC composites are promising materials for reactor core structures, and of course, more research is needed. Thus there appears to be a high likelihood that the use of composites would lower the costs of some Generation IV reactor designs, and could prove advantageous for the PBR and could be highly advantageous for the LFTR. Carbon-carbon composites could also be expected to lower reactor costs, and increase transportability of large reactor modules or even complete small reactors, by lowering reactor weight.

It should be noted that that this discussion points to the compatibility of advance materials with generation IV reactors, and in some cases their incompatibility with other well regarded reactor designs. Thus an unexpected consequence of the potential advantages of advanced materials would be a preference for certain Generation IV reactors - the PBR and the LFTR - over LWRs, and LMFBRs.

I would like to express my appreciation to the participants in the Energy From Thorium Engineering Materials Forum for advancing my knowledge and stimulating my thinking about advanced materials. Of course, I picked up the ball and as usual ran with it in a different direction. The Forum members are not responsible for any fumble I might have incurred.

3 comments:

The most obvious trade off like that for LFTRs is one I believe the French are looking into. Instead of providing a distinct moderator, say of graphite complete with supporting structure and possible degradation over time, just provide a large enough reactor vessel and enough fuel material for the fluorine, lithium and beryllium to do enough moderating.

I have seen a statement made some 10 years ago about manufacturing difficulties with SiC/SiCs, but I am not aware if the problem was due to a lack of experience, or if it reflected an underlying problem with SiC/SiCs.

I would withhold any judgment in silicon carbide. It's probably the most promising avenue for high fluence composites but SiC/Sic and SiC matrix carbon composites are a really new technology.

As I understand, one of the main difficulties is a lack of good SiC precursors for the composite's matrix. SiC fibers are more routine.

Most SiC "matrices" are actually porous pyrolytic carbon matrices infiltrated with silicon precursors then pyrolized a second time. The final matrix is not really SiC but a mishmash of sub-stoichiometric Si(x)C(y) with a structure closer to the original pyrolytic carbon than to true nanocrystalline or amorphous SiC. In large pieces, the SiC layer tends to be superficial, rather heterogeneous and with a lot of defects from venting the pyrolysis by-products and from macro-crystallization under strong pyrolysis conditions (needed to fully convert the precursors).

It looks like the situation is getting better now with polymers like polyalkylsilanes (forms that don't spontaneously burst in flame, that is ...) and polyalkylsilynes. But 1) those developments are very recent and still very much in the lab (polymethylsilynes are not even 5 years old) and 2) those molecules are mostly geared toward fiber forming and thin coating, not massive piece manufacturing so there is no experience applicable to nuclear reactors yet.